To optimise diesel engine combustion, it is necessary to have precise control over the quantities of fuel delivered by the fuel injectors. It is desirable to be able to inject small quantities of fuel across a wide range of fuel pressures. For heavy-duty applications in particular, the fuel injectors must be capable of delivering fuel in small quantities at very high fuel pressures. Furthermore, to meet increasingly stringent emissions targets, it is desirable that the timing of each injection and the quantity of fuel delivered with each injection can be controlled with great accuracy.
Typically, a fuel injector includes an injection nozzle having a valve needle which is movable towards and away from a valve needle seating so as to control fuel injection into the engine. The valve needle may be controlled by means of a nozzle control valve, which controls fuel pressure in a control chamber for the valve needle.
In one known type of injector, the nozzle control valve includes a control valve member which is moveable between a first position, in which fuel under high pressure is able to flow into the control chamber, and a second position in which the control chamber communicates with a low-pressure fuel reservoir, such as a low-pressure fuel drain. A surface associated with the valve needle is exposed to fuel pressure within the control chamber such that the pressure of fuel within the control chamber applies a force to the valve needle to urge the valve needle against its seating.
In order to commence injection, the nozzle control valve is actuated such that the control valve member is moved into its second position, thereby causing fuel pressure within the control chamber to be reduced. The force urging the valve needle against its seating is therefore reduced and fuel pressure within the delivery chamber serves to lift the valve needle away from its seating to permit fuel to flow through the injector outlet. In order to terminate injection, the valve arrangement is actuated such that the control valve member is moved into its first position, thereby permitting fuel under high pressure to flow into the control chamber. The force acting on the valve needle due to fuel pressure within the control chamber is therefore increased, causing the valve needle to be urged against its seating to terminate injection.
Small and accurately controllable injection quantities can be achieved by reducing the opening rate of the valve needle whilst maintaining a high closing rate. In other words, an asymmetric control arrangement is desired, in which the pressure in the control chamber decreases relatively slowly to achieve a controlled needle lift when the control valve member is moved into its second position, but increases relatively quickly to terminate injection rapidly when the control valve member is moved into its first position.
One way of achieving an asymmetric opening and closing characteristic is to provide an additional, direct flow path for supplying high-pressure fuel to the control chamber. This additional flow path is not under the control of the control valve, and serves to increase the pressure in the control chamber more rapidly during needle closure. Such arrangements are described in EP 1 541 860 B and EP 1 988 276 B.
Another way of achieving an asymmetric opening and closing characteristic is to modify the nozzle control valve to define a restricted flow path for fuel flow between the control chamber and a low pressure drain.
EP 1 604 104 B and EP 1 835 171 B describe restricted flow path arrangements that achieve asymmetric control. In each case, a restricted flow path is provided in the form of an annular clearance between the outer surface of the control valve member and the internal surface of the bore within which the control valve member is received. The restricted flow path restricts the rate of flow of fuel from the control chamber to the low-pressure drain, and therefore damps the opening movement of the valve needle, but does not hinder the re-filling of the control chamber with high-pressure fuel to allow rapid needle closure.
For increased emissions control, fuel injection strategies increasingly include multiple small pre- and/or post-injections of short duration and low volume, and it is desirable to increase further the ability of fuel injectors to deliver such short injections accurately. One problem with existing control valve arrangements utilising drain restrictions in the form of clearances between parts is that, due to manufacturing tolerances, it is relatively difficult to ensure that the cross-sectional area of the restricted flow path is accurately and consistently dimensioned during manufacture. Furthermore, in use, the cross-sectional area of the restricted flow path tends to vary with temperature, resulting in inconsistent operation.
There is also a trend towards increasing fuel injection pressures, for example to increase fuel atomisation and penetration. For this reason, fuel may be supplied to the injector at very high pressures (of the order of 3000 bar or more). The fuel pressure in the control chamber and in the associated passages can therefore approach or reach this high pressure when the valve needle is closed. When the valve needle is open, the fuel pressure in the control chamber and the associated passages drops significantly. Accordingly, it is desirable to ensure that the injector and, in particular, the passages associated with the control chamber, are able to withstand cyclical high-pressure loading. Furthermore, it is also desirable to reduce the risk of component damage due to cavitation and other fluid-flow effects that may occur as the high-pressure fuel in the control chamber is released to drain.
It is also desirable to reduce the part count and manufacturing complexity of fuel injectors to reduce cost and improve reliability.
Against this background, it would be desirable to provide a fuel injector having a control valve arrangement which allows more accurate control of the valve needle movement, whilst minimising part count and manufacturing complexity and reducing the risk of injector failure.